Ethylene is known to be a regulator of programmed cell death (PCD) during plant development (Campbell and Drew, Planta 157:350-357 (1983); Drew et al, Planta 147:83-88 (1979); He et al., Plant Physiol. 112:1679-1685 (1996)) and plays a role in orchestrating PCD in developing cereal endosperm: exogenous ethylene can accelerate the onset of the cell death program in developing endosperm whereas inhibitors of ethylene biosynthesis or perception delay the program (Young et al., Plant Physiol. 119:737-751 (1997); Young and Gallie, Plant Mol. Biol. 39:915-926 (1999); Young and Gallie, Plant Mol. Biol. 42:397-414 (2000)). Ethylene controls many aspects of plant growth and development such as fruit development, root and leaf growth and seed germination.
Ethylene perception involves membrane-localized receptors that, in Arabidopsis, include ETR1, ERS1, ETR2, ERS2 and EIN4 (Chang et al., Science 262:539-544 (1993); Hua et al., Science 269:1712-1714 (1995), Hua et al., Plant Cell 10:1321-1332 (1998), Sakai et al., Proc. Natl. Acad. Sci. USA 95:5812-5817 (1998)). ETR1, ETR2 and EIN4 are composed of three domains, an N-terminal transmembrane ethylene binding domain (Schaller and Bleeker, Science 270:1809-1811 (1995)), a histidine protein kinase domain, and a C-terminal receiver domain. ERS1 and ERS2 lack the receiver domain. These genes have been grouped into two subfamilies based on homology, where ETR1 and ERS1 comprise one subfamily and ETR2, ERS2, and EIN4 comprise the other (Hua et al., Plant Cell 10:1321-1332 (1998)). ETR1 and ERS1 both contain a functional histidine kinase domain and autophosphorylate histidine residues, whereas other family members lack particular residues required for histidine kinase activity. However, in vitro studies indicate that ETR2, ERS2, and EIN4 are capable of autophosphorylation on serine and threonine residues (Moussatche and Klee (2004), J. Biol. Chem., 279:48734).
Only two ethylene receptor genes have been identified in maize (i.e., ZmETR2 and ZmERS1) in contrast to the five types identified in Arabidopsis (Gallie and Young (2004) Mol Genet Genomics 271: 267-281). ZmETR2 has two variants, named ZmETR9 and ZmETR40. Members of both ethylene receptor families are expressed to substantially higher levels in the developing embryo relative to the endosperm. This explains why, despite the fact that the endosperm and embryo each contribute to the synthesis of ethylene, the developing endosperm exhibits a low threshold to ethylene-induced cell death while the embryo is protected.
The endosperm of cereals serves as the major storage organ for grain but undergoes cell death during mid to late seed development. Ethylene regulates the timing of the onset of cell death in the developing endosperm. Because ethylene is a gas that can pass freely through membranes, all organs of the developing kernel might be expected to be exposed to ethylene generated by a specific organ and diluted only by their distance from the generating source. The ability to limit cell death to specific organs within the developing kernel suggests tight control of the expression of the ethylene biosynthetic and perception machinery.
The role of ethylene in photosynthesis is unclear. Photosynthesis is often inhibited during conditions of stress, e.g., drought, ozone exposure, or chilling (Flexas and Medrano (2002), Annals Bot. 89: 183-189; Chaves et al. (2002), Annals Bot. 89: 907-916; Ramachandra et al. (2004), J Plant Physiol. 161: 1189-1202). Photosynthetic capacity increases during leaf expansion and declines with leaf age until it reaches low levels prior to the onset of leaf senescence (Gay and Thomas (1995), New Phytol. 130: 159-168). The rate of initiation and execution of a senescence program significantly impacts the ultimate contribution that a leaf can make to a plant. This is of particular relevance to those crops, such as cereals, where yield potential is reduced by adverse environmental conditions that induce premature leaf senescence.
The effect of ethylene on photosynthesis has been controversial with reports suggesting either no effect or an inhibitory effect (Pallaghy and Raschke (1972), Plant Physiol. 49: 275-276; Kays and Pallas (1980), Nature 285: 51-52; Pallas and Kays (1982), Plant Physiol. 70: 598-601; Squier et al. (1985), Environ Sci Technol 19: 432-437; Taylor and Gunderson (1986), Merr. PlantPhysiol. 86: 85-92; Woodrow et al. (1988), Mill J. Exp Bot. 39: 667-684). The difference in species, growth conditions, intact versus excised leaves, and concentration of exogenous ethylene used may have contributed to the variation in observed effects. A mutant approach was employed to examine the effect of ethylene on photosynthetic activity and grain development in maize (Young et al. (2004), Plant J 40: 813-825). The authors in this study found that maize mutants with defective ethylene production had increased amounts of chlorophyll and rates of CO2 assimilation relative to wild type plants.
Because ethylene plays such a large role in plant growth and development, the identification of genes involved in the ethylene response pathway is useful for creating plants with phenotypes associated with an altered ethylene-related process, such as plants having staygreen traits. Accordingly, a need exists for the identification of genes involved in cereal ethylene signal transduction pathways.